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LM5118: Output drops under load

Zach Olson
Genius 11855 points
Part Number: LM5118
Other Parts Discussed in Thread: LM5155

I am using the LM5118 as a phantom power supply in an audio application. The power supply circuit has the following specifications:

Vin = 5V

Vout = 52V

I(min) = 2.2mA

I(max) = 10mA

Switching Frequency = 400kHz

The circuit also has an LDO that drops the 52V to 48V. Then I have a switch that will apply the 48V to power an LED and a condenser microphone. I used a design from Webench which does provide me with 52V under no load.

However, when a load is applied, in this case just the LED which is only using 2.2mA, the output of the LM5118 drops to 42V.

I have tried using various inductor values and the corresponding comp loop values with no success.

I am not sure why this circuit can't keep up with such a small load?

  • 0
    TI__Guru* 78645 points

    Hi Zach,

    Thanks for posting.  Why use a buck-boost when the Vin=5V and Vout=52V?  A boost seems to be a better choice.  You should consider the LM5155, our latest boost/sepic/flyback controller. 

    Anyway, it seems you hit the max duty cycle.  Please reduce the switching frequency to 100kHz. 

    Thanks,

    Youhao Xi, Applications Engineering